1. Field of the Invention
This invention relates to telephone subscriber instruments and in particular to such instruments wherein the conventional electromechanical arrangements are replaced with electronic circuits.
2. Description of the Prior Art
The conventional electromechanical telephone instrument is gradually being replaced by devices containing electronic circuitry. The electromechanical arrangement, while reliable, is expensive to manufacture and does not lend itself readily to use with a modern electronic exchange. In particular, a number of facilities that can be provided by an electronic exchange cannot be utilized by the conventional instrument.
Electronic instruments require a degree of complexity according to the particular customer requirement. The simplest requirement is for the basic conventional service in what is commonly known as a "plain ordinary telephone (POT)". Other customers require varying degrees of sophistication to take advantage of the range of facilities offered by the electronic exchange.
This range of operational requirements has in the past caused some difficulty in the design of the instrument. Either a single complex device has been used involving a high degree of redundancy for POT applications or different circuits have been specifically designed for each degree of service required. These practices are of course relatively costly. Furthermore, the high current drain introduced by the addition of audio amplifier circuits for hands-free operation may reduce the available voltage to a level below that at which the control circuitry will operate reliably.
A further difficulty with telephone circuits arises from the different requirements of the various circuit portions. Thus the logic and control functions are more suited to CMOS techniques whereas the analog functions are better performed by bipolar circuits. As it is at present impractical to integrate both CMOS and bipolar devices on a single chip it is preferable to divide the various operating functions between a plurality of chips to take full advantage of the features of the two technologies. In present arrangements, however, one chip from such a chip set cannot be used singly, i.e., the entire chip set must be installed even where all the functions thus provided for are not required.
An object of the present invention is to minimize or to overcome these disadvantages.